In the design of a multiple component assembly, a primary concern of the designer is the fastening of the piece parts or components that comprise an overall assembly. The ease with which plastic parts can be molded into a wide variety of configurations and the relative elasticity of thermoplastics allow the designer to incorporate structural elements used in the attachment of the parts integrally into the piece part itself. Examples include the use of snap-fit type latches, the inclusion of a receiving boss for accepting a threaded screw, and structures comprising a hinge. Many different design configurations for plastic hinges incorporated into plastic piece parts are known. See, e.g., U.S. Pat. No. 2,948,430, issued Aug. 9, 1960 to W. D. Teague, Jr. et al., U.S. Pat. No. 4,549,670, issued Oct. 29, 1985 to Trendler, U.S. Pat. No. 3,333,726, issued Aug. 1, 1967 to P. J. Belanger, and U.S. Pat. No. 2,732,581 issued Jan. 31, 1956 to E. R. Heck.
A plastic molding technique that has become quite popular recently is blow molding. In blow molding, melted plastic is extruded through a dye to form a hollow shell (or "parison") with a specific wall thickness distribution. The parison is lowered between the core and the cavity halves of a mold. The mold halves close upon the parison; simultaneously, air or another suitable gas is injected into the interior of the parison to force the parison to stretch and conform to the mold surface. The result is a plastic part that has an external skin in the desired shape and a virtually empty internal core consisting of air pockets created by the injected gas. Blow molding can be advantageously used for large parts, which would require extremely high injection pressures if injection molded, and for thick parts, since the air pockets, or "voids", present in the core of the part affect the strength and rigidity of the part very little but reduce the amount of material required to make the part considerably.
For all of its advantages, blow molding is beset by some practical operational limitations. One of those limitations is the difficulty and expense of using "side-action" mechanisms within the mold. Side-action mechanisms create apertures and other structures oriented perpendicularly to the direction of mold travel by moving perpendicularly in response to the opening and closing of the mold. Side action mechanisms are particularly useful for molding integral fastening structures, such as receiving sockets for hinges, that are not oriented in the direction of mold travel. Exemplary is a hinged "clam shell" type enclosure: the mold core and cavity close and separate in the direction parallel to the sidewalls of the enclosure, so an integral hinge socket for fastening one half of the clam shell to its mating half is typically oriented perpendicular to the direction of mold travel. As such, the hinge socket must be formed with a retracting side action mechanism. In blow-molding, the use of side-action mechanisms is particularly undesirable, as the operation of a side action mechanism in a blow-molding mold requires that the portion of plastic the side action is to act upon must be stretched not only in the direction of mold travel, but also in the direction of the side action perpendicular to the direction of mold travel. This stretching thins that portion of plastic considerably, often so much that it completely loses its physical integrity and fractures. In addition, the inclusion of a side action mechanism in a mold increases the complexity of that mold; with increasing complexity comes increased cost for fabrication and maintenance. Because integral hinge structures typically require side-action mechanisms, blow-molded hinge structures are virtually unknown.
As a consequence, hinge structures included as part of an integral plastic design that do not require a mold with a side-action mechanisms are desirable, as such a design would facilitate the use of blow molding to make hinging parts. In view of this, it is an object of the present invention to provide a hinge design that can be molded integrally with the hinging parts without the use of side actions in the mold. It is a further object that such a design be easily assembled and have sufficient integrity as to be essentially permanent.